Insulin-like growth factor-I (IGF-I) is a 70-amino acid, single-chain peptide and a member of the insulin family of peptides, which also includes insulin and insulin like growth factors II (IGF-II ) and III (IGF-III). IGF-I plays an essential role in growth and development of many tissues (Liu et al., Cell : 59-72, 1993; Baker et al., Cell 75: 73-82, 1993). In the central nervous system, IGF-I is expressed during fetal development of the brain and peripheral nerves (Werner et al., Proc. Natl. Acad. Sci. USA 86: 7451-7455, 1989; de Pablo et al.,TINS 18: 143-150, 1995). Both in-vitro and in-vivo studies have demonstrated that IGF-I and IGF-II promote motor neuron survival, proliferation and neurite outgrowth (Caroni, P., et al., J. Cell Biol., 110: 1307-1317, 1990). In-vitro studies have shown that IGF-I promotes the survival of astrocytes and neuronal precursor cells from fetal rat brain and motor neurons (Ang et al., J. Neurol. Sci. 109: 169-172, 1992; Komoloy et al., Proc. Nat;. Acad. Sci. USA 89:1894-1898, 1992; Hughes et al., J. Neurosci. Res. 36:663-671, 1993; Gammeltoft et al., pp.295-305, in The Insulin-like Growth Factors and Their Regulatory Proteins, Elsevier Science B.V., Amsterdam, The Netherlands, 1994 editors, Baxter et al.). In-vivo studies have demonstrated that local infusion of IGF-I to the proximal end of a cut sciatic nerve promotes regeneration of the peripheral nerve (Nachemson et. al., Growth Factors 3: 9-314, 1990). Furthermore, recent studies demonstrate that repeated (chronic) systemic administration of recombinant human insulin-like growth factor-I (rhIGF-I) to mice enhances the functional recovery of the sciatic nerve following bilateral crush of sciatic nerves (Contreras et al., J.Pharmacol. and Exp. Therapeut. 274(3): 1443-1449, 199). Taken together, these studies have suggested that IGF-I has therapeutic value in certain pathological conditions involving motor neurons such as peripheral neuropathies (peripheral neuropathy generally refers to a disorder that affects the peripheral nerves, most often manifested as one or a combination of motor, sensory, sensorimotor, or autonomic neural dysfunction), ALS and other neurological diseases. (Lewis et al., Ann. N.Y. Acad. Sci. 692: 201-208, 1993. See also, U.S. Pat. No. 5,093,317 and U.S. Pat. No. 5,420,112). Pre-clinical studies have also shown that IGF-I promotes neuronal sprouting (the natural process by which neurons generate additional branches, enabling them to establish functional contacts with muscle fibers whose original nerve contacts have been lost as a result of neuronal death) and function of peripheral nerves, and induces skeletal muscle hypertrophy, or enlargement of muscle cells, in the presence of neurodegenerative conditions.
U.S. Pat. No. 5,420,112 discloses the use of IGF-I to treat peripheral nerve damage, called peripheral neuropathy, caused by peripheral-neuropathy-causing toxic agents. ALS is a fatal neuromuscular disease characterized by the chronic, progressive degeneration of motor neurons which leads to muscle weakness, muscle atrophy, and eventually death from respiratory failure. U.S. Pat. No. 5,093,317 discloses the use of IGF-I in the treatment of certain disorders such as ALS. Cephalon Inc.'s Phase III clinical studies of MYOTROPHIN.RTM. (rhIGF-I) have shown that patients with ALS who received MYOTROPHIN recombinant protein experienced less disease severity, slower progression of disease, and better functional ability compared to patients who received placebo. After nine months of therapy, patients who received 0.10 mg/kg per day of MYOTROPHIN recombinant protein showed approximately 25 percent less deterioration than patients receiving placebo. The data demonstrate statistically significant effects of MYOTROPHIN recombinant protein on ALS disease severity and progression. The data also suggest that the effects of MYOTROPHIN recombinant protein administration are dose-dependent. In these studies, continuous, daily, injection of IGF-I for the life of the patient is contemplated.
While the results of the cited clinical trials establish that administration of IGF-I deters the effects of neurological diseases, such as ALS, there is no rapid, immediate method to determine whether a patient is receiving a therapeutically effective dose of IGF-I for the treatment of the disease or condition being treated. Determining an effective per patient dose of IGF-I is important for a beneficial therapeutic regimen, as a patient may not reap the complete benefits of the administered drug if a less than effective dose is administered. Furthermore, a beneficial therapeutic outcome may not result if a dose of IGF-I that is chronically administered results in biochemical tolerance. Ideally, it would be beneficial to both a physician and a patient receiving IGF-I if a Surrogate Biochemical Marker (SBM) within the patient could be assessed to determine if an effective dose of the therapeutic has been administered.
An SBM is a measure of some parameter associated with, but not a direct measure of, drug efficacy. Whereas physiologic markers (endpoint measurements, such as disease episodes, quality of life measures, mortality, etc.) do not always provide the level of accuracy and precision required to detect small changes and thus make resulting data of limited value, SBMs can provide a specific and graded quantitative measurement of a drug's effect on the body's response within a short time. Thus, SBMs can be useful for determining the appropriate dose regimen for each particular patient and as an aid in ensuring that the patient is receiving a therapeutically effective dose of a therapeutic drug such as IGF-I. For a review on SBMs, see Lee et al., J. Clin. Pharmacol. 464-470, 1995.
In response to either an initial (acute) or repeated (chronic) administration of rhIGF-I, plasma glucose concentrations decrease. This is consistent with previous reports which show that IGF-I has insulin-like activity on blood glucose concentrations (Snyder et al., J. Clin. Endocrinol. Ans Metab. 71: 1632-1636, 1990; for review see Froesh et al., Ann. Rev. Physiol. 47: 443-467, 1985). Thus, plasma glucose might be considered a possible SBM. However, changes in plasma glucose concentrations are not useful as a SBM for determining whether a patient has been given a therapeutically effective dose of IGF-I because several other factors, including circadian rhythms, nutrition, activity and stress levels also effect plasma glucose concentrations.
High affinity insulin-like growth factor binding proteins (IGFPBs) are important modulators of cellular responsiveness to IGF-I. (Clemmons, Growth Regulation, 2: 80-87, 1992). Insulin-like growth factor binding proteins (IGFBPs) are thought to facilitate transport of IGF-I, to modulate the actions of IGF-I on target cells and to control IGF transport in blood and out of the vascular compartment, localizing and modulating IGFs to specific cell types and binding to receptors and regulating blood glucose levels. (Lewitt et al.,Mol. Cell Endocrinol. 79: 147-152, 1991; Lewitt et al., Endocrin. 129: 2254-2256, 1991, Holly, Acta Endocrinol. 124: 55-62 1991; for review see Clemmons, Mol. Reprod. and Develop. 35: 368-375, 1993). While 95% of plasma IGF-I is associated with binding with IGFBP-3, the remaining 5% of IGF-I has been shown to be bound to IGFBP-1, -2, and -4. A study in transgenic mice that over expressed the IGF-I gene led the authors to conclude that those mice have greater levels of IGFBP-2 and IGFBP-3, and that IGF-I is a major controller of these binding proteins (Camacho-Hubner et al., Endocrinology 129: 1201-1206, 1991). Another study by Clemmons et al., (J. Clin Endocrinol Metab. 727-733, 1991) determined the nutritional and hormonal variables that regulate IGFBP-2 in humans, reporting that plasma IGFBP-2 levels were not suppressed in acromegaly, post prandially, after administration of growth hormone, acute stimulation of insulin secretion or after glucose infusion. Extreme insulin deficiency, however, resulted in a 1.7 fold increase in IGFBP-2.
The IGFBPs in mammalian plasma range between 24 kDa and 55 kDa with respect to their molecular weights. IGFBP-3 appears as a doublet (48-55 kDa) presumably due to different glycosylation states; IGFBP-2 is a 34-36 kDa protein; IGFBP-1 has a molecular mass (M.sub.r) of 31 kDa and IGFBP-4 appears at 24 kDa (Camacho-Hubner et al., 1991, supra).
IGF-I is highly conserved across mammalian species. For example, there are only minor differences between the amino acid sequences of rat and human IGF-I. The observed differences between rat and human IGF-I are relatively few and mostly conserved in nature; there are no differences between bovine, porcine, and human IGF-I (Daughaday et al., Endocrine Rev., 10(1):68-91, 1989). As such, and beneficially, those in the art have utilized recombinant human IGF-I (rhIGF-I) for in vivo investigations in animal models, including mice and rats, indicating the highly conserved nature of the protein and its receptor.